High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same composition by solid state polymerization. The low molecular weight polyesters which are used in such solid state polymerizations are generally prepared by conventional melt polymerizations. Solid state polymerization is generally considered advantageous in that the handling of high molecular weight ultra-high viscosity molten polymers during the polymerization phase is eliminated. Thermal degradation is also essentially avoided during the solid state portion of the polymerization.
The low molecular weight polyesters utilized in solid state polymerizations are generally in the form of pellets or chips. Such pellets can vary greatly in size; however, as a general rule, the smaller the size of the pellets of polyester the faster the solid state polymerization will proceed. Very fast rates of solid state polymerization can be attained by utilizing polyesters which are in the form of porous pills.
Most thermoplastic polyesters, including polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), produced by melt-phase polymerization are almost completely amorphous in nature. Such amorphous polyesters which are prepared by melt polymerization are normally crystallized from the amorphous state to the crystalline state prior to solid state polymerization to prevent the polyester pellets or chips from sticking, clumping, or agglomerating during solid state polymerization.
When an amorphous polyester is heated from ambient temperature to above its glass transition temperature (Tg) during a standard crystallization process, it will soften and become sticky at the onset of crystallization. After the polyester pellet is sufficiently crystallized, it looses its stickiness and becomes more rigid. The sticking temperature of an amorphous polyester is usually about 20.degree. C. above its Tg. The crystallization rate of the polyester will not be fast enough to be practical until its temperature is further raised to about 30.degree. C. above its sticking temperature. To achieve the maximum crystallization rate, the temperature of the polyester must be raised even higher. For example, PET has a Tg of 74.degree. C. and a sticking temperature of about 95.degree. C. The crystallization rate of PET is rather low until the temperature is raised to above 125.degree. C., and in practice, PET is usually crystallized at temperatures between 150.degree. C. and 190.degree. C.
PEN has promising properties for fiber and packaging applications. PEN has a Tg of about 118.degree. C. and a crystalline melting point T.sub.m of 268.degree. C. It exhibits a crystallization peak between 180.degree. C. and 220.degree. C. Its sticking temperature is about 140.degree. C. when in the amorphous state. According to conventional wisdom, the best crystallization temperature range for PEN polymer would be between 180.degree. C. and 220.degree. C.
During the crystallization process, the PEN polyester undergoes a sticky stage. This takes place within the period of time the polyester temperature exceeds the sticking (softening) temperature and the time the polyester becomes well crystallized. To mitigate the agglomeration and lumping effect as the PEN polyester pellets pass through their sticky stage, commercial-scale crystallizers for continuous crystallization of polyesters can be equipped with means to provide vigorous agitation. In a batch process, a variable speed, variable temperature rotating vessel or a fluidized bed can be used. With respect to PET polyester polymers, two types of continuous crystallizers have been widely used, namely, agitated vessels and fluidized beds.
Heretofore, in the continuous crystallization process of particulate polyesters, PET in particular, polyester pellets are charged at ambient temperature directly into the crystallizer without any pretreatment. The heat transfer medium in a crystallizer used in a continuous process is generally hot air, hot nitrogen, or hot oil in order to subject the polyester pellets to a fast rate of temperature rise and to effect a fast rate of crystallization. Under appropriate operating conditions, the PET polyester pellets can be crystallized without lumping or agglomeration.
Unlike PET pellets, however, when PEN pellets are exposed to crystallization conditions herein the rate of temperature rise toward the crystallization temperature climbs quickly, the pellets "popcorn" by undergoing a sudden and rapid expansion as they are heated to near the crystallization temperature. The puffed up skins of the pellets are very sticky and, within seconds, the pellets agglomerated tightly into big lumps, vigorous agitation notwithstanding. The popcorning phenomena indicates that those conventional PET crystallization processes in which the rate of temperature rise in a crystallizer is high, are not suitable for those desiring to crystallize PEN polyesters in large scale commercial production.
Without being bound to a theory, we believe the cause for the sudden expansion of PEN pellets during cyrstallization heat-up is that the total internal vapor pressure of volatile material (such as water) within the pellet exceeds atmospheric pressure when the temperature reaches the softening point of the PEN pellet. Once the pellet is softened, the pressurized vapors of volatiles within the pellet can expand and puff the softened PEN pellet. Two reasons exist for the high vapor pressure of the volatiles at the softening temperature of PEN pellets. First, because of the high barrier properties of PEN, the removal of volatiles from inside the pellet during the temperature rise from ambient temperature to the sticky temperature is slow. When the pellet reaches the sticky temperature, a substantial amount of volatiles remain trapped within the pellet. Second, the softening temperature of PEN is high, eg in excess of 140.degree. C. The combination of PEN's high sticky temperature and the substantial amount of volatiles remaining within the pellet results in the total vapor pressure within the pellet exceeding atmospheric pressure. The popcorning effect is not observed in PET pellets because the sticky temperature of PET is around 95.degree. C., which is much lower than the sticky temperature of PEN and below the boiling point of water. Further, the barrier properties of PET are lower than that of PEN. Accordingly, the combination of lower barrier properties and the lower softening temperature of PET prevents the total internal vapor pressure from exceeding atmospheric pressure, and what little volatiles remain within the PET pellet as the crystallization temperature rises from 95.degree. C. through the boiling point of water to 110.degree. C. are insufficient to generate a total internal vapor pressure which can distort the shape of pellet which has developed rigidity during the crystallization cycle. The sudden expansion of PEN pellets during heat up to crystallization is discussed in U.S. Pat. No. 4,963,644. According to this patent, the cause for the sudden expansion of PEN pellets during crystallization was investigated by subjecting a PEN pellet to a DTA scan at a scan rate of 10.degree. C./min. Its DTA thermogram exhibited an endotherm near the onset of the crystallization exotherm. The endotherm was believed to arise from the sudden vaporization and/or release of volatiles, whose total internal vapor pressure exceeded atmospheric pressure, trapped inside the pellet as the PEN is softened near its crystallization temperature. This phenomenon explained the sudden expansion of PEN pellets as they were exposed to standard crystallization temperatures of 180.degree. C. to 220.degree. C.
The volatile material trapped inside of PEN pellets arise from a number of different sources, such as contaminants entering the process during melt polymerization or the formation of by-products generated during melt polymerization. Due to the higher temperature at which the melt of PEN polymer is held during melt polymerization compared to the temperature at which PET is held during melt polymerization, the number and quantity of by-products generated in PEN melt polymerization is greater than in PET melt polymerization. Degradation of PEN could generate water, ethylene glycol, acetaldehyde and the like. Because of the very high melt viscosity of PEN, these by-products are difficult to remove during pelletizing. Furthermore, PEN is often pelletized under nitrogen pressure. In this case, nitrogen could also be trapped inside the pellets. PET, on the other hand, generates fewer by products, is more stable in its melt state, and its melt viscosity is lower than that of PEN. The quantity of by-products generated in PET are relatively small and are more easily removed during pelletizing. Therefore, very little volatile material is trapped inside PET pellets to cause lumping and sticking problems during crystallization.
The solution proposed in U.S. Pat. No. 4,963,644 to the lumping and sticking problem of PEN pellets during crystallization was to slowly remove the volatiles trapped inside the pellets at temperatures below its sticking temperature prior to the crystallization step. This process incorporated a devolatilization step before the crystallization step. This patent calls for a devolitilization step involving (1) heating the amorphous polyethylene naphthalate to a temperature which is within the range of about 80.degree. C. to about 140.degree. C. in the presence of a stream of inert gas or under a vacuum to devolatilize the amorphous polyethylene naphthalate; and (2) subsequently heating the devolatilized polyethylene naphthalate to a temperature which is within the range of about 150.degree. C. to about 260.degree. C. while providing agitation to produce the crystallized polyethylene naphthalate.
It would be desirable, in the interest of improving the processing speed, to find a way to avoid the need for subjecting the PEN pellets to the slow devolitilization step without sacrificing the advantage of abating the tendency of the pellets to lump together into a large mass. We would desire to rapidly heat up PEN pellets during the heat up phase in a crystallization step comparable to the rapid heat up of PET pellets in crystallizers, such as fluidized or agitated bed crystallizers, without experiencing the popcorning effect.